1. Field of the Invention
The present invention relates to a method and apparatus for dressing a polishing cloth, and more particularly to a method and apparatus for dressing a polishing cloth for restoring the polishing capability of the polishing cloth in a polishing apparatus for polishing a workpiece such as a semiconductor wafer having a device pattern thereon to a flat mirror finish by bringing the surface of the workpiece into contact with a surface of the polishing cloth.
2. Description of the Prior Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at photolithographic process can form interconnections that are at most 0.5 μm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish, them with a polishing apparatus, and such a process is called Chemical Mechanical Polishing (CMP). In CMP the semiconductor wafers are chemically and mechanically polished while supplying an abrasive liquid comprising abrasive grains and a chemical solution such as an alkaline solution.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotating. In the conventional polishing apparatus, a nonwoven fabric cloth is often used as a polishing cloth for polishing the semiconductor wafer having a device pattern thereon.
However, the recent higher integration of IC or LSI demands a more and more planarized finish of the semiconductor wafer. In order to satisfy such a demand, harder materials, such as polyurethane foam, have been recently used as the polishing cloth. After, for example, one or more semiconductor wafers have been polished by bringing the semiconductor wafer into sliding contact with the polishing cloth and rotating the turntable, abrasive grains in the abrasive liquid or ground-off particles of the semiconductor wafer are attached to the polishing cloth. In the case of the nonwoven fabric cloth, the polishing cloth is napped. In the case where the semiconductor wafers are repeatedly polished by the same polishing cloth, the polishing performance of the polishing cloth is degraded, thus lowering the polishing rate and causing a nonuniform polishing action. Therefore, after polishing a semiconductor wafer or during polishing of a semiconductor wafer, the polishing cloth is processed to recover its original polishing capability by a dressing process.
For a dressing process for recovering the polishing capability of the polishing cloth made of relatively hard material such as polyurethane foam, there has been proposed a dresser having diamond grains. This dressing process using the diamond grain dresser is effective in restoring the polishing capability of the polishing cloth and tends not to rapidly lower the polishing rate thereof.
To be more specific, the dressing process is classified into two processes, one of which is a process for raising the napped polishing cloth by a blush, water jet or gas jet and washing out the remaining abrasive grains or the ground-off particles from the polishing cloth, and the other of which is a process for scraping off a surface of the polishing cloth by diamond or SiC to create a new surface of the polishing cloth. In the former case, even if the dressing is not uniformly performed over the entire dressing area of the polishing cloth, the polished surface of the semiconductor wafer is not greatly affected by the thus dressed polishing cloth. However, in the latter case, the polished surface of the semiconductor wafer is greatly affected by the polishing cloth which has been nonuniformly dressed.
Conventionally, the polishing apparatus having a diamond grain dresser comprises a top ring for holding the semiconductor wafer and pressing the semiconductor wafer against a polishing cloth on a turntable, and a dresser for dressing the surface of the polishing cloth, the top ring and the dressing being supported by respective heads. The dresser is connected to a motor provided on the dresser head. The dresser is pressed against the surface of the polishing cloth while the dresser is rotated about its central axis and the dresser head is swung, thereby dressing a certain area of the polishing cloth which is to be used for polishing. That is, the dressing of the polishing cloth is preformed by rotating the turntable, pressing the rotating dresser against the polishing cloth, and moving the dresser radially of the polishing cloth by swinging the dresser head. In the conventional dressing process, the rotational speed of the dresser is equal to the rotational speed of the turntable.
However, when the polishing cloth is dressed by the diamond grain dresser, the polishing cloth is slightly scraped off. Unless the polishing cloth is uniformly scraped off in any vertical cross section, i.e., is uniformly scraped off in a radial direction of the polishing cloth, the semiconductor wafer, which is a workpiece to be polished, cannot be uniformly polished as the number of dressing processes increases. It is confirmed by the inventors of the present application that when the dressing is performed in such a manner that the rotational speed of the dresser is equal to the rotational speed of the turntable, the amount of material removed from the inner circumferential region of the polishing cloth is greater than the amount of material removed from the outer circumferential region of the polishing cloth.
FIG. 6 shows measurements of the removal amount of material in the polishing cloth which has been dressed by the conventional dressing method. In FIG. 6, the horizontal axis represents a distance from a center of rotation, i.e., a radius (cm) of the polishing cloth, and the vertical axis represents the amount of material removed from the polishing cloth, which is expressed by a removal thickness (mm) of material. FIG. 6 shows measurements of the removal thickness were the same and about 500 semiconductor wafers were polished on the polishing cloth and the corresponding number of dressing processes were applied to the polishing cloth. Two kinds of diamond grain sizes were used in the experiment. For example, the rotational speed of the turntable was 13 rpm, the rotational speed of the dresser was 13 rpm, 500 semiconductor wafers were polished on the polishing cloth made of polyurethane foam, and a corresponding number of the dressing processes were applied to the polishing cloth. In this case, the difference in a removal thickness of material between the outer circumferential region and the inner circumferential region of the polishing cloth was about 100 μm.